From the archive, 12 August 1930: Pluto: the new planet

Friday 12 August 2011 09.34 EDT
First published on Friday 12 August 2011 09.34 EDT

It would seem all doubts as to the existence of the new planet announced from the Lowell Observatory in March last have been set at rest. Fifteen years ago the late Dr. Percival Lowell published his "Memoir on a Trans Neptunian Planet," in which he not only demonstrated that such a planet must exist but that certain small "residual perturbations" of the planet Uranus were satisfactorily explained by its existence. He also computed for the new planet a period of about 282 years, a distance of over 3,720,000,000 miles from the sun, a mass seven or eight times that of the earth, and an apparent brightness of the twelfth or thirteenth stellar magnitude. He further showed that the new planet was likely to be located in one or other of two regions of the sky – Gemini or Sagittarius.

For years the planet has been searched for at the Lowell Observatory. Not till early this year did the Lowell astronomers succeed in detecting in the constellation Gemini a faint moving star-like point, which seemed to answer the description of the long-looked-for planet, and on March 13, Dr. V. M. Slipher, Dr. Lowell's successor as director of the observatory, felt sufficiently confident to announce the discovery of Lowell's trans-Neptunian planet. However, doubts began to accumulate as to whether the newly discovered body was really Lowell's planet. That it was a moving body belonging to the solar system no one could deny, but it might be in reality a very distant comet. A planet as massive as that postulated by Dr. Lowell – a body with a diameter of about 16,000 miles – ought to be considerably brighter than the fifteenth magnitude, unless its albedo, or reflective capacity, were very low, which is highly improbable.

Meanwhile astronomers began to examine photographs of the Gemini region of the sky taken over a period of years, with a view to finding the image of the planet. On a plate taken at the Uccle Observatory in Belgium on 27 January 1927, an image was detected which bore a striking resemblance to the new planet, and which has been, with great probability, presumed to be actually the planet. Making use of the Uccle observation, Dr. Crommelin was able to compute an orbit for the planet which turned out to be surprisingly similar to the orbit calculated by Lowell. Images of the new planet have since been found on plates taken at Mount Wilson in December, 1919.

Dr. Crommelin at the June meeting of the Royal Astronomical Society said: "At the April meeting we thought Lowell's prediction was a fluke, but I do not think so now.'' The one error made by Lowell was to overestimate the mass. Dr. Slipher has given the new body the name Pluto..